The present invention relates to electroluminescent (EL) devices, such as self-light-emitting elements, that can be used as light sources for flat-type self-light-emitting display devices for purposes such as communications and illumination. For example, the flat-type self-light-emitting elements of the present invention may be used in automotive instrument display panels. More particularly, the present invention relates to materials used in such EL devices.
In general, an EL device is a spontaneously luminous element characterized by injecting electrons and electron holes into an inorganic or organic layer kept between electrodes, and recombining them in a luminous layer. It is important to efficiently recombine the carriers (electrons and holes) in the luminous layer. EL devices are broadly divided into two types: inorganic and organic.
The inorganic type EL device is further divided into two types: dispersion and thin-film. In each of these types, a high-voltage alternating current (AC) is disadvantageously required to drive the devices. Additionally, the red color is weak and the number of colors is small. Further, although the dispersion type EL device can be manufactured by a relatively easy and low-cost method, such as screen printing, its useful life is short. Thus, it has been put to practical use as a backlight in a wrist watch or the like. In contrast to the dispersion type of EL device, the thin-film type requires a dry manufacturing method, such as vacuum deposition, having facilities similar to those for semiconductor manufacture. Therefore, this type of EL device is costly to produce. Nonetheless, Nippondenso Co., Ltd. has put such an EL device to practical use in a vehicle, and Sharp Corp. has put it to use in an electric scoreboard. Costs are further increased for both types of inorganic EL devices because they require a costly high-voltage AC drive circuit.
The organic type of EL device is also divided into two types: low-molecular and high-molecular. In each of these types, a low-voltage direct current (DC) is required to drive the devices. Further, red, green, and blue, colors reach levels that can be used practically. With respect to the driving principles and the structures, no difference exists between the low-molecular type EL device and high-molecular type EL device. However, the high-molecular type EL device can be manufactured by a wet type technique, such as spin coating, whereas the low-molecular type is manufactured by vacuum deposition or the like as in the case of the inorganic thin-film type EL device. Pioneer Electronic Corp. has put the low-molecular organic EL device to practical use as a display for a vehicle-mounted audio system, whereas N. V. Philips"" Gloeilampenfabrieken has put the high-molecular organic EL device to practical use as a segment type display for a portable telephone.
Although each of these types of EL device has achieved some practical use, there is room for improvement with respect to driving voltage requirements, useful life, brightness or luminous efficiency, and manufacturing techniques. By luminous efficiency, it is meant the ratio of an amount of input current to an amount of output light. If the current input to the EL device, i.e., the carriers, are all converted into visible light, an internal efficiency is 100%. However, the recombining of holes and electrons actually takes place outside of the luminous layer so that much of the carriers are converted into thermal vibrations or energy other than visible light. Thus, the internal efficiency for most EL devices is actually around several percent.
The main causes for deterioration of the organic EL element are the following: penetration into, and attachment to, the element by the active molecules of water, as well as the change in properties of the element itself.
An object of the present invention is to decrease the required driving voltage for EL devices. Another object of the present invention is to increase the useful life of EL devices. Still another object of the present invention is to enhance the brightness, or luminous efficiency, of EL devices. Another object of the present invention is to simplify the manufacturing technique for EL devices. Yet another object of the present invention is to more uniformly disperse the doping nanostructures within the layers of EL devices.
The above and other objects of the invention can be achieved by doping nanostructures into the layers of EL devices and, in particular, into the luminous layer of EL devices. Further, the present invention uniformly distributes the doping nanostructures through the various layers that make up EL devices.
The nanostructures may be fullerenes, single walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs), fullerenes, porphorines, metal filled nanotubes, and the like. Further, the carbon nanotubes may be shortened, long, doped with boron, doped with nitrogen, or pure. Especially, B-doped carbon nanotubes and N-doped carbon nanotubes can be used to better match the energy levels of the polymer with the ionized potential (IP) of the nanotube. In the present specification, the term xe2x80x9ccarbon nanotubexe2x80x9d includes carbon nanotubes and doped carbon nanotubes and structures. Further, boron nitride nanotubes may be used as the nanostructures of the present invention, i.e., instead of, or in addition to, the carbon nanostructures. As used herein, and in accordance with that well known in the art, the term xe2x80x9cfullerenesxe2x80x9d includes carbon cluster compounds represented by C60, C70, C76, C78, C82, C84, C90, C96, and C140, for example.
Examples of the host include PPV, MEH-PPV, DOOHPPV, POMPV, PmPV, PFO, PFO-red, PFO-blue, PANi, PP, and so forth.
More particularly, the present invention includes doping carbon nanostructures into PPV, especially by first combining the carbon nanostructures with a PPV derivative. That is, a first substance is formed comprising carbon nanostructures and PPV containing carbazole units, wherein the carbon nanostructures and PPV containing carbazole units are present in a 1:1 ratio. The PPV containing carbazole units preferably is poly [(2,5-di-pentoxyl-phenylene)-4-diylvinylene-3,6-(9-(1-azafulleroid-propyl) carbazolenevinylene)], and the carbon nanostructures are preferably fullerenes, whereby the first substance is PPV-AFCAR.
Then, an amountxe2x80x94ranging from greater than 0 wt % to 3 wt %xe2x80x94of the first substance is then combined with a second PPV. More specifically, this second PPV preferably is poly{1,4[2-(3,7-dimethyloctyloxy)3,5,6-trimethoxy] phenylene vinylene} or POMPV, whereby a compound for use as a layer of an EL device is formed. This compound includes many advantages over the related art.
First, doping carbon nanostructures into the layers of EL devices reduces the required driving voltage. Second, doping carbon nanostructures into the layers of EL devices leads to a longer useful life for the EL device. Third, doping carbon nanostructures into the layers of EL devices enhances the brightness of the EL devices. More specifically, the present invention achieves a higher luminance and external efficiency due to enhanced recombination dynamics. Fourth, doping carbon nanostructures into the layers of EL devices leads to simplified manufacturing techniques.
Further, although doping carbon nanostructures into the layers of EL devices has many advantages, if the carbon nanostructures aggregate, they can lead to current leaks. Such current leaks degrade the performance of the EL device as a whole. Therefore, it is important to uniformly disperse the carbon nanostructures throughout the layer of the EL device into which they are doped. In order to more uniformly disperse the carbon nanostructures in a layer of an EL device, as well as to obtain a good connection between the dopant carbon nanostructures and the host material, the carbon nanostructures are first compounded with, or attached to, a derivative of the layer into which they will be doped, thereby forming a first substance. This first substance, including the carbon nanostructures, is then doped into the material that forms the desired layer in an EL device. It is easy to form the desired layer of the EL device in this manner, therefore EL devices of the present invention easily can be synthesized in large quantities for industrial utilization.
In particular, the above and other objects can be achieved by providing a compound, for use in an EL device, comprising from greater than 0 wt % to 3 wt % of a first substance comprising about 50 wt % carbon nanostructures and about 50 wt % of a PPV derivative, and the remainder comprising a second PPV derivative.
Additionally, the above and other objects can be achieved by providing a compound for use in an EL device, comprising a first substance comprising a carbon nanostructure covalently linked to a PPV molecule containing a carbazole unit; and a second substance comprising a PPV derivative different from the PPV molecule containing a carbazole unit.
Further, the above and other objects can be achieved by providing an EL device comprising a cathode; an anode; a light emitting layer disposed between the cathode and the anode, whereby when a potential is applied across the cathode and the anode, the light emitting layer emits light, and wherein the light emitting layer comprises a compound as set forth above.
Moreover, the above and other objects can be achieved by providing a method of making an EL device, comprising providing a substrate; covering the substrate with a buffer layer; depositing a layer of a compound onto the buffer layer, wherein the compound is as set forth above, and acts as an emissive layer; disposing an electron transport layer on the emissive layer; and disposing a cathode on the electron transport layer.
Other features and aspects of the present invention are discussed in greater detail below.